Combustors are commonly used in industrial and commercial operations to ignite fuel to produce combustion gases having a high temperature and pressure. For example, gas turbines and other turbomachines typically include one or more combustors to generate power or thrust. A typical gas turbine used to generate electrical power includes an axial compressor at the front, multiple combustors around the middle, and a turbine at the rear. Ambient air enters the compressor as a working fluid, and the compressor progressively imparts kinetic energy to the working fluid to produce a compressed working fluid at a highly energized state. The compressed working fluid exits the compressor and flows through one or more fuel injectors in the combustors where the compressed working fluid mixes with fuel before igniting to generate combustion gases having a high temperature and pressure. The combustion gases flow to the turbine where they expand to produce work. For example, expansion of the combustion gases in the turbine may rotate a shaft connected to a generator to produce electricity.
At particular operating conditions, combustion dynamics at specific frequencies and with sufficient amplitudes, which are in phase and coherent, may produce undesirable sympathetic vibrations in the turbine and/or other downstream components. In the context of this invention, coherence refers to the strength of the linear relationship between two (or more) dynamic signals, which is strongly influenced by the degree of frequency overlap between them. Typically, this problem is managed by combustor tuning which limits the amplitude of the combustion dynamics in a particular frequency band. However, combustor tuning may unnecessarily limit the operating range of the combustor.
Altering the frequency, coherence, phase, and/or amplitude of the combustors may reduce unwanted vibrations of the turbine and/or other downstream components. One approach to reducing unwanted combustion-driven vibrations in downstream components is to alter the coherence of the combustion system. For instance, as the frequency of the combustion dynamics in one or more, but not all, combustors is driven away from that of the other combustors, coherence and, therefore, modal coupling of the combustion dynamics of the combustors is reduced, which, in turn, reduces the ability of the combustor tone to cause a vibratory response in downstream components. Alternatively, shifting the combustion dynamics frequency of each of the combustors away from the natural frequency of the downstream components may also reduce unwanted vibrations of downstream components.
Therefore, a system and method for operating a gas turbine that detects the possibility of unwanted vibrations in downstream components and/or reduces such unwanted vibrations by altering the frequency, phase, amplitude, and/or coherence between combustors would be useful for enhancing the thermodynamic efficiency of the combustors, protecting against accelerated wear, promoting flame stability, and/or reducing undesirable emissions over a wide range of operating levels, without detrimentally impacting the life of the downstream hot gas path components.